Re: Pac repair using a chip comparitor

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The Bugtrap or HP Logic Comparators work pretty well as long as you
remember that it works by comparing the OUTPUTS of the IC under test. If
there is a problem with an input that is external to the IC under test
(ICUT) then the comparator will NOT show a fault even if the ICUT has a bad
output-assuming the fault on the input causes the same output to be in the
same state in the Test IC. For example if a 7404 (hex invertor) gate has a
shorted to ground output, and the input is stuck high for that gate, then
the comparator will show that there is no problem...(with the output)...

In fact ALL a comparator shows you is that the outputs match the input
logic, nothing else! Still this is VERY handy if you also use your logic
probe to check that the inputs are working as they should.

John :-#)#

At 01:46 AM 12/12/00, mike_ranger@deface.ca wrote:
>Hi all
> I was working in the shop last night on a pac that has been giving
>me fits. I tried the "staring at it for hours" approach first, but that did
>nothing to fix it :-0
>
> Weird symptoms. With a standard set up, I was getting a screen full
>of slashes and zeros, flipping every now and again. I replaced the vram
>with one of Clay's most excellent products, and the board now went through
>self test over and over. up to, but not including the cross hatch. Self
>test showed nothing. Removing the two middle roms changed nothing as well.
>(that was a clue). I tried a special daughter card I have that runs MS pac
>with no roms on row 6. booted to currupt screen images, so I felt there was
>a problem in adressing the roms, or ram.
>
> Last night, on a whim, I decided to try out my new HP 10592 logic
>comparitor. I set it up to check 8k becuase I noticed the IRQ was not
>firing. The comparitor said it was fine. Since it was my first crack at
>using the tool, I was not convinced. Going back to the adressing issue, I
>decided to check the ROM selct lines, by 7N chip, 74ls42. The comparitor
>showed 3 bad outputs on this. Swapped it and the board came to life. Even
>works with the original vram, so the bad chip could have been messing with
>the RD signal. No sound yet, but I'll get that working :-)
>
> On the Logic comparitor. The HP is a nice little unit, but you must
>get one with the zif card. This was an add on produced in later runs. It
>allows you to configure your chip with dip switches, where the original unit
>shipped with cards you had to phsically alter to match the chip
>characteristics.
>
> Others in the comunity feel he BUGTRAP device is a better
>comparitor, but I have no experience with it, so if you are looking for one,
>you should investigate those as well.
>
> The logic comparitor will be pretty much useless on any tri-stated
>device. It will not make a newbie a guru, and like any test equipment, you
>have to understand the it's limitations to use it effectively.
>
> Too bad I go sniped on that fluke model 90 z-80 thingy last friday
>:-( Gusee I'm going to have to save up for one of them nifty 9010A's
>
>Mike
>
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Received on Mon Dec 11 08:09:52 2000